Recently, color image forming apparatuses adopting electrophotography, inkjet printing, and the like require higher resolution and higher image quality. In particular, the tonality of a formed color image and the stability of density in a formed image greatly influence the image forming characteristics of the color image forming apparatus. It is known that the density of an image formed by the color image forming apparatus varies upon a change in environment or long-time use. Especially an electrophotographic color image forming apparatus loses the color balance of a formed image upon even small density variations, and efforts must be made to always keep its density characteristics to tonality constant. For this purpose, the color image forming apparatus comprises a tonality correction means (e.g., look-up table: LUT) for correcting, for toner of each color, image data and process conditions such as several luminous exposures and several bias voltages for development in accordance with different absolute temperatures and humidities. The color image forming apparatus selects process conditions optimal for the environment and the optimal value of tonality correction on the basis of an absolute temperature/humidity measured by a temperature/humidity sensor.
In order to obtain constant density characteristics to tonality even upon variations in the characteristics of each part of the apparatus, the following density control is performed. First, a patch image for detecting density is formed on an intermediate transfer material, photosensitive drum, or the like with toner of each color. Then, the density of the unfixed toner image is optically detected by a density detection sensor. Process conditions such as the luminous exposure and the bias voltage for development are determined on the basis of the detection result (see Japanese Patent No. 3,430,702).
In density control (to be referred to as single-color control hereinafter) using the density detection sensor, a patch image is formed on an intermediate transfer material, photosensitive drum, or the like, and the density of the patch image is detected, but a change in the color balance of an image obtained by subsequently transferring and fixing a toner image onto a transfer material is not detected. The color balance changes depending on the transfer efficiency of transferring a toner image onto a transfer material and the heating and press for fixing. Such change cannot be dealt with by the above-mentioned density control using the density detection sensor for detecting the density of unfixed toner.
To solve this problem, the following color image forming apparatus has been proposed. A density or chromaticity detection sensor (to be referred to as a color sensor hereinafter) for detecting the density of a single toner image on a transfer material (sheet) or the chromaticity of a full-color image after transferring and fixing the toner image onto the transfer material is arranged on the downstream side of a fixing unit. An output from the color sensor is fed back to, e.g., a look-up table (LUT) for correcting image data and process conditions such as the luminous exposure and the bias voltage for development, and the density or chromaticity of an image formed on a transfer material is controlled. The color sensor uses light sources for emitting red (R), green (G), and blue (B) beams as light emitting devices in order to identify C, M, Y, and K colors and detect the density or chromaticity. Alternatively, the color sensor uses a light source for emitting a white (W) beam as a light emitting device, and three types of filters having different spectrum transmittances for red (R), green (G), blue (B), and the like are formed on a light sensor. By three outputs, e.g., R, G, and B outputs from the color sensor, C, M, Y, and K signals are generated and the density of an image can be detected. The chromaticity of an image can be detected by performing a mathematical process such as linear transform for R, G, and B outputs or conversion on the basis of the look-up table (LUT).
Various methods have conventionally been proposed for controlling the density or chromaticity of a formed image. For example, the following method has been proposed as a prior art of changing the gamma conversion characteristic on the basis of a density obtained by measuring a formed image, or correcting a color matching table or color separation table on the basis of a measured chromaticity. This method detects the chromaticities of a black single-color tone patch and CMY mixed-color tone patch on a transfer material by using a color sensor for detecting the chromaticity of a transfer material and that of a patch formed on the transfer material. The chromaticities of these two tone patches are compared, and when they coincide with each other, it is determined that the CMY mixed-color tone patch is achromatic and the lightness of the CMY mixed-color tone patch is equal to that of the black single-color tone patch (see Japanese Patent Laid-Open No. 2003-084532). Further, a color image forming apparatus has been proposed which calculates from the color identification result the mixture rate at which a CMY mixed-color tone patch becomes achromatic, and keeps the density characteristics to tonality constant. This method can advantageously correct variations in the spectral characteristics of the color sensor because the CMY mixture rate is determined on the basis of the spectral reflectance characteristics of black.
However, in control (Japanese Patent Laid-Open No. 2003-084532) of adjusting CMY-mixed gray to the chromaticity of black (K), at least a K density control table must be updated before control using the color sensor, and preliminary single-color control is indispensable. When the updated density characteristics to tonality for K are not proper, i.e., the lightness of K serving as a reference varies to a non-negligible degree (only the lightness varies and a color difference ΔE permissible to a human being becomes ΔE>3), the lightness of CMY-mixed gray varies following K variations. As a result, the characteristics of color process and halftone characteristics deviate from the density characteristics to tonality of each color that are set by the design.